Abstract

Neurons and glia (astrocytes and oligodendrocytes) are the two major cell types that make
up the central nervous system (CNS). They are generated from precursor domains within
the neuroepithelial germinal zone (ventricular zone, VZ) that surrounds the ventricles of the
brain and the central canal of the spinal cord (the embryonic neural tube). In general,
neurons are generated before glia. The intra-spinal circuits that control movement and
locomotion are made up of different neuronal and glial elements that develop separately
but come together to form interconnected functional units. To understand the logic of
circuit development and ultimately circuit-driven behaviour, it is necessary to understand
where and when each type of cell originates. To identify the products of the most ventral
progenitor domain in the developing spinal cord, known as (Nkx2.2-expressing p3 domain), I
made use of Cre-loxP technology. I generated a transgenic mouse line that expresses an
inducible form of Cre recombinase (CreERT2) under Nkx2.2 transcriptional control and
crossed this with a Cre-dependant reporter mouse to visualize p3-derived progeny. I
confirmed that the p3 domain generates Sim1-expressing V3 interneurons, serotonergic
interneurons as well as visceral motor neurons of the hindbrain. p3 progenitors also produce
two spatially restricted subtypes of astrocytes, a few oligodendrocytes and ventrallypositioned
ependymal cells. Unexpectedly, my studies also revealed that pre-ganglionic
motor neurons of the sympathetic nervous system (SPNs, visceral motor neurons of the
thoracic spinal cord), as well as a population of dorsally-located Sim1-expressing
interneurons, are produced from Nkx2.2-expressing precursors. SPNs have been generally
believed to originate from the same progenitor pool as HB9-positive somatic motor neurons
(sMNs), defined by expression of Olig2 (pMN domain, immediately dorsal to p3). Supporting
this idea, no spinal sMNs or SPNs are formed in Olig2-null mice. However, I found that
Nkx2.2-expressing p3 precursors do not generate any HB9-positive sMNs, implying that sMNs
and SPNs derive from distinct precursors - the latter from the most ventral part of the pMN
domain that transiently co-expresses Nkx2.2 and Olig2. Thus, segregation of SPNs and sMNs
occurs already in the neuroepithelium before their post-mitotic progenitors migrate away
from the VZ into the ventral horns. This is how visceral and somatic MNs are known to
develop in the brainstem, so my results provide a unifying theme to MN development at
different levels of the neuraxis.